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From: Watson 'Atto Parsec' Name
Subject: Re: Best way to power array of LEDs?
References: <0001HW.B9C26E7400A7161B165FEAC0@news.covad.net> <2Qan9.3129$cS4.firstname.lastname@example.org> <3DC843FE.4A25FFF3@fanwap.com> <3DC99AD4.2AB0FE52@fanwap.com>
X-Newsreader: MicroPlanet Gravity v2.60
Date: Thu, 7 Nov 2002 05:07:07 -0800
NNTP-Posting-Date: Thu, 07 Nov 2002 06:08:09 PST
Organization: InReach Internet
In article <3DC99AD4.2AB0FE52@fanwap.com>, email@example.com talked about...
> Yes, I mean in series.
> But two strings can not use the same R values.
> So sad it very hards to calculate the current and R.
> the forward voltage drop does not identical in each LED.
> and it changes when current change too.
> strings 1 : ( 12 volt - ( 3.453 + 3.6324 + 3.833 ) ) / 40 ohm = 27 mA
> Now I want 20 mA , How to calculate the resistor ?
> Since forward voltage change when the current change too.
Early this year or last year we had this newsgroup discussion about making
a simple current regulator for LEDs. You will have to put your LEDs in
series in place of the single LED and adjust the resistor values for the
higher voltage. For a start you will probably want to put a pot in place
of the 2.2k resistor. Here's the simple version of the schematic. To view
this you must use Courier font.
Rich Grise wrote:
> If I were building it, I'd lose the 2N3904, both of the 1N914's,
> the 3.3K, and the 22K, and just connect the wiper of the pot to
> the base of the 2N4401. You might want to add a zener across the
> pot to regulate its voltage as the batteries droop, but I'm way
> too lazy to calculate that for you. In fact, I've used silicon
> diodes in series, forward biased, as more-or-less of a voltage
> regulator. Oh! I see - you've already got D1 forward-biased.
> Yeah, the 2N3904 has way too much gain for what you're trying
> to accomplish. Put a couple or 3 more 1N914's in series with
> D1, and put the rest of the circuit back in your parts bin. :-)
Thanks for the idea, I just might tack solder a couple parts together to
see how it works. I'm really not concerned with the current regulation
being precise. All I want to do is eliminate the wide variation of the LED
current with the changing battery voltage abd minimize the dropout. In
other words I don't wanna use a fifth AA cell to increase the 'stiffness'
of the current using just a resistor to limit the current.
Do you mean something like the following???
> Watson A.Name wrote:
> > Current Limiter, Low Dropout, for LEDs
> > by Watson A.Name Jan 16, 02 rev 020116A
> > + = connection ) = no connection
> > Must be viewed with monospaced font such as Courier
> > +---------------+--------------+
> > | | |
> > \ | |
> > 2.2 / --- White |
> > k \ \ / LED |
> > / --- |
> > | | |
> > +---+ | |
> > | | | |
> > | | | / |
> > | \ |/ ----- four
> > --- / 470 +----| 2N4401 === Ni-
> > \ / \ | |\ ----- MH
> > D1 --- / | |E \ === AA
> > 1N | | | | ----- cells
> > 914 | +-------+ | === 4.0
> > D2 | | +-o T.P. ----- to
> > | | | === 5.5V
> > --- \ \ -----
> > \ / / 2.2k / 10 |
> > --- \ \ ohms |
> > | / / |
> > | | | |
> > +---+---------------+--------------+
Jan 20, 2002 Measurements taken with B&K 2707 DMM.
At 5.5V, T.P. V = .325V. At 4.0V, T.P. V = .271V.
This is about 20 percent variation in LED current over the full range.
This is not as good as the original adapted circuit that had about 13
percent over the full range. But it represents a considerable improvement
over a single resistor, which had a 3:1 variation in current over the full
V range. I think that this circuit is a good compromise between
simplicity, parts count and parts availability, and compactness since there
will be a circuit for each LED in a multiple LED light.
> Lizard Blizzard wrote:
> > Nucharin W. Jansen wrote:
> > > What's happen when voltage drop across LEDs are not same ?
> > > I have problem with white LEDs in series.
> > > I can't control the current.
> > > I have to adjust in real work one by one in each branch.
> > You say the LEDs are in series, so the current is identical in each one.
> > All you have to do is put a current limiting resistor in series with
> > those, and use a value that will give you the right current thru all.
> > If you made a mistake and meant parallel, then yes, you should not
> > connect LEDs in parallel without a resistor in series with each LED to
> > limit and equalize the current.
> > [snip]
My email address is whitelisted. *All* email sent to it
goes directly to the trash unless you put NOSPAM in the
Subject: line. alondra101 hotmail.com
Don't be ripped off by the big book dealers. Go to the URL
that will give you a choice and save you money(up to half).
http://www.everybookstore.com You'll be glad you did!
Just when you thought you had all this figured out, the gov't
changed it: http://physics.nist.gov/cuu/Units/binary.html
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